Improvements in the performance of focus detection systems in recent years have been accompanied by many proposals relating to so-called multi-point focus detection systems of multi-point focusing type in which range finding is possible at multiple points within a viewfinder. In a multi-point detection apparatus that performs multi-point focusing by utilizing the principle of phase-difference autofocusing, use is made of a method that involves acquiring a pair of image signals corresponding to respective focus-measurement points, performing a correlation computation and computing a defocus amount from the phase difference between the signals.
The specification of Japanese Patent Application Laid-Open No. 5-11178 discloses a technique for selecting defocus amounts, which are in an order designated by counting from the short-distance side, from among effective defocus amounts obtained from multiple lines or areas.
The specification of Japanese Patent Application Laid-Open No. 6-138378 discloses a method of selecting a focus-measurement point that conforms to the intent of the operator based upon an output from line-of-sight detecting means and the reliability thereof.
The specification of Japanese Patent Application Laid-Open No. 2006-145862 discloses a method of determining the similarity of a plurality of pairs of image signals and, in a case where similarity is high or identical defocus amounts are obtained, combining a plurality of image signals, generating pairs of image signals and performing a correlation computation.
The specification of Japanese Patent Application Laid-Open No. 2002-258142 discloses a sensor in which, when a signal representing a subject is small, sums signals in a direction that is perpendicular to a pupil dividing direction and reads out the sum.
In general, a focus detection apparatus is required to have certain functions. For example, besides accuracy at the time of focusing, one function is the ability to capture the main subject suitably in scenes where subjects are at long and short distances, and another function is the ability to perform focus detection in situations of diminished brightness.
In order to achieve the ability to detect focus under conditions of diminished brightness, hardware-based improvements are conceivable. These include raising the sensitivity of the focus detection sensor, lengthening the time over which the signal from a focus detection sensor is accumulated and adopting a divided-pupil arrangement in which the quantity of light that is incident upon a focus detection sensor is increased. In addition, a software approach for synthesizing a focus detection sensor signal has also been adopted. In a case where a focus detection sensor signal is synthesized, often a pair of image signals is obtained by combining a plurality of pairs of image signals in a direction that is perpendicular to the pupil dividing direction. Such processing makes focus detection possible under conditions of diminished brightness while suppressing degradation of image signals in the direction of correlation. On the other hand, when such processing is executed, there are instances where an inappropriate result of computation is obtained if the same subject does not appear at each pixel that is to undergo synthesis. Further, if an image that can be approximated by a line that is oblique with respect to the direction of correlation appears, there are instances where it is difficult to obtain a correlation from the result of addition.